In thin film electronic devices fabrication, the process comprises depositing different layers of materials on a substrate. For example, in conventional amorphous silicon solar cells on silicate glass substrates, the layers comprise (1), a conductive layer, either metallic and therefore opaque, or transparent and conductive, such as of indium tin oxide, (2) a p-type amorphous silicon layer, (3) an intrinsic amorphous silicon layer and (4) an n-type amorphous silicon layer. A major disadvantage of the glass substrate is that it is not only fragile, but also for a large area device an additional protective frame has to be employed with a concomitant increase in the cost of the device. Furthermore, since the glass itself is not conductive, a layer such as (1) described above is necessary. This conductive layer is usually a crystalline form and therefore, not amorphous. The growth of amorphous material on a non-amorphous substrate leads to a phase instability and therefore, is not desirable.
A metallic substrate, such as steel, is often used as a replacement for the glass substrate. In this case, steel directly, or steel with a passivation metal layer such as aluminum, titanium or tungsten can be used as an electrode. Since steel and metal coated steel have a crystallite structure they will have many grains and grain boundaries. Along the boundaries, sharp spikes and cavities exist. A non-uniform thickness or disrupted surface is formed when the size of these local irregularities is the same order of magnitude as the thickness of the film. This type of surface deteriorates the device performance. Processes to polish away the spikes or the cavities are very laborious and costly and, therefore, not practical. Metallic glass eliminates the problems associated with metal substrates and avoids the necessity of a separate conductive layer in the case of glass substrates. Metallic glass' electrical conductivity and amorphous morphology permit the direct deposition of an amorphous semiconductor material thereon.
In some film devices, which as gallium arsenide and related III-V compound semiconductors, cadmium sulfide and related II-VI compound semiconductors, or germanium and selenium elementary semiconductors, crystallization can be achieved at a low temperature and is independent of the morphological nature of the substrate. In this case, metallic glass also can be used as a substrate.
Metallic glass, because it is readily producible in large quantities on the order of tens of kilometers in the form of a very thin, flexible ribbon, and because of its elastic nature in this ribbon form, provides an ideal substrate for many electronic devices. However, it should be understood that metallic glass in rigid form is also useful as a substrate for semiconductor devices.